The present invention relates to a process sealed microwave waveguide for use with a tank level sensor. The process sealed waveguide serves as a pathway for transmitting microwave energy from a microwave source through an explosion proof/flame proof process seal interface to an antenna. This device is particularly useful in instruments that determine product levels in tanks.
Radar level gauges are in wide use for measuring the level of products in tanks. Process fluids contained in tanks range from benign products to severely corrosive or abrasive compounds. The operating restriction that is present is that the product contained in the tank must have a dielectric constant high enough to cause microwave signal reflection which can be received by the gauge. Antenna and waveguide assemblies that are used also have to be capable of containing the process pressures in the tanks in order to isolate the gauge electronics from the process vapors and fluids.
It is important that the level gauge waveguide contain process pressures both in terms of preventing damage to the gauge electronics and in terms of preserving the tank's ability to contain process pressures. An additional design constraint for microwave level gauges used in process applications that are considered to be hazardous locations are explosion proof/flame proof certification requirements. These requirements come from standards developed by various representatives of industry, government, and certifying agencies. The need for containing an internal explosion of a specific flammable vapor-air mixture and not allowing the propagation of ignition temperature gasses to the external environment is set forth in the standards. At the present time, meeting the design requirements of these explosion proof/flame proof standards has required very precise manufacturing processes and the addition of high precision components. This results in greater labor and material costs to be placed on an instrument already significantly constrained by microwave performance criteria that must be met for proper gauge operation. More specifically, the waveguide or pathway from the electronics or microwave source must include an explosion proof/flame proof seal as well as serve as a microwave signal pathway.
One existing solution to the problem includes separating the gauge into two assemblies: one being the antenna/microwave sensor assembly and the other the electronics assembly. During installation of this gauge, the antenna/microwave sensor assembly is assembled to the tank with a separate process seal. The electronics assembly contains the circuitry for power regulation, signal processing, communications, and the level and volume calculations and corrections. The antenna/microwave sensor assembly is a very low power device that transmits and receives microwave signals. The spacing between the components used in the antenna/microwave sensor assembly electronics and the low power level permit the assembly to meet the requisites for intrinsically safe instrumentation. However, even with the separated level gauge components there is still a need for the process seal between the sensor and the potentially aggressive products inside the tank.
Another method of avoiding the explosion proof/flame proof requirements at the interface of the source of the microwave signal and the antenna, is to encapsulate the antenna inside an agency approved epoxy, RTV or glass. In order for this to be feasible the antenna must be small or the additional epoxy required and extensive housing modifications would make the gauge design quite expensive. When the antenna is encapsulated inside epoxy, the customer has two mounting options, namely the microwave level gauge can be mounted directly on the tank or the gauge can be mounted on a "window" covering an opening or port in the tank. Both the window and the gauge must be designed to meet material, safety certification, microwave performance, and process seal requirements.
However, the need still exists for a low cost, effective, and high performance process sealed waveguide assembly that fulfills the microwave signal transmission requirements, acts as a process seal, and meets the explosion proof/flame proof certification requirements.